The Korean War resulted in one of the four greatest population movements of Koreans. The first of these was Korean emigration to Manchuria and Japan during the Japanese colonial period; by 1945 there were between 1.5 and 2 million Koreans in each of Manchuria and Japan (for a total of over 10% of the Korean population). The second of these population movements was the return of many of these emigrants after 1945. About 80% of Koreans in Japan returned to Korea, and about 40% of those in Manchuria. Most returned to the provinces in Korea they had originally come from. (The difference in return rate is because Koreans in Manchuria were given Chinese citizenship, while Koreans in Japan were denied Japanese citizenship and faced significant discrimination, and many had been forced to move to Japan.) In total, about 1.8 million Koreans returned to South Korea, mostly from Japan. Fewer Koreans returned to North Korea since (a) fewer had emigrated from the provinces in North Korea, and (b) Manchurian Koreans from the north were less likely to return to Korea than Manchurian Koreans from the south.
The third large population movement was from North to South in response to the division of Korea and the North Korean Revolution. Approximately 740,000 Koreans moved to the South, and there was only a very small movement from South to North.
The fourth and last of these population movements was the wartime movement. The front lines swept over most of the country twice (and four times in some areas, including Seoul), so there was plenty of opportunity for the movement of people. The number is not known exactly, but is estimated at about 650,000 moving from North to South, and about 290,000 from South to North. There was a big difference between the two movements: the North-to-South movement included almost as many women as men (the male:female ratio was 1.2; many moved as families), while the South-to-North movement was over 90% male. This suggests that a large part of the South-to-North movement might have been involuntary movement due to conscription into the North Korean army.
About half of the wartime refugees moving to the South settled in the two northernmost provinces of South Korea, while about 20% moved to the far south-east of South Korea (mostly to Busan), perhaps to get as far away from the North as possible.
For more detailed data, see: